1. Field of the Invention
The present invention relates to a method of determining valve events to optimize engine operating parameters for a given engine speed/load point.
2. Background Art
An engine equipped with a variable valve timing system has the potential to effectively manage exhaust gas re-circulation without the need for external equipment and to effectively control the cylinder charge, thus determining the operating point of the engine. Variable valve timing systems provide an opportunity to select the best valve timing events for a desired engine operating condition. However, given a variable valve event timing system capable of this flexibility, a valve timing event control method providing a comprehensive valve timing event strategy is necessary for these potentials to be realized.
Such a desired comprehensive valve timing event strategy would control the aspiration of the engine without the need for external gas re-circulation or throttling while providing the highest level of exhaust gas residual reasonable for a desired engine operating condition, minimizing NOx formation, and maximizing fuel economy. In essence, the desired comprehensive valve timing event strategy would manage internal exhaust gas re-circulation and cylinder air charge for an engine equipped with a variable valve timing event system.
In general, the valve timing event control method providing the desired comprehensive valve timing event strategy would implement real time valve timing event control as a mechanism for managing cylinder charge, thereby eliminating the need for a conventional throttle body which is a source of considerable pumping losses. Thus, given an engine without a throttle, a camshaft drive mechanism, and an external exhaust gas re-circulation equipment, the desired comprehensive valve timing event strategy would ideally optimize fuel economy; minimize emissions; not preclude implementation of other advanced control strategies; be conductive to continuous, transient engine control; be generic enough to be easily applied to any naturally aspirated four stroke engine; and make physical sense.
Accordingly, it is an object of the present invention to provide a method for determining valve events to optimize engine operating parameters for a given engine speed/load point.
In carrying out the above objects and other objects, the present invention provides a method for determining valve timing events to optimize operating parameters of an engine having a variable valve timing system over an entire operating range of the engine. The method includes the steps of (A) setting combinations of intake valve opening and closing timing events and exhaust valve opening and closing timing events for a given engine speed; (B) for each combination, obtaining a fuel conversion efficiency measure and an emissions measure of the engine at the given engine speed; (C) selecting combinations which optimize a weighted cost function of the fuel conversion efficiency measure and the emission measure for a range of engine output torque settings at the given engine speed; (D) repeating steps A, B, and C for each given engine speed across a range of engine speeds; and (E) assembling a feed-forward map based on the selected combinations for the entire operating range of the engine.
The optimization algorithm employed by the method generally includes the following steps. First, at each engine speed, the intake valve opening and closing timing events (IVO, IVC) and the exhaust valve opening and closing timing events (EVO, EVC) are swept. The spark and fuel are adjusted to maintain the best mean engine torque output (i.e., engine load) and the desired air-fuel ratio at each valve setting. The next step is that at each engine speed/load point, the valve timing events are selected from all possible valve timing event combinations which minimize a cost function of fuel consumption and emissions. From a local optimization perspective, one approach is to choose the minimum NOx (i.e., emissions) production allowable without degradation of combustion stability, sacrificing some degradation in fuel conversion efficiency. Globally, a later trade-off may be made between emissions and fuel economy and a cycle basis.
Unfortunately, accurate NOx formulation from simulation is difficult as is predicting combustion stability. So instead, in simulation, burnt exhaust gas residual is used as an indicator of NOx (cylinder temperature could be used alternatively). In the laboratory, a NOx measurement and an indicator of combustion stability, such as covariance of indicated mean effective pressure, could be used to refine the optimization.
The next step is to determine the valve timing events to obtain the desired engine load and the desired exhaust gas residual. It is desirable and sometimes necessary to have high exhaust gas residual at low engine loads (for desirable NOx emissions), low exhaust gas residual (for good performance), and a smooth transition in between. It is also desirable to have minimal exhaust gas residual at engine idling for proper combustion stability.